Reflective optical elements, which are used in optical systems that operate with high light intensities and/or with light in the very short-wave spectral range such as the VUV or EUV spectral range, are exposed to high thermal loads during operation. Examples are reflective optical elements which are used in optical systems for material processing with laser light with a very high laser power, for example for cutting, drilling, welding, soldering or fusing. Further examples are reflective optical elements which are used in optical systems that are used in the very short-wave spectral range, in particular in the EUV or even in the X-ray region. A specific example is a collector mirror which is used in the EUV light source of a microlithographic projection exposure apparatus.
On account of the high thermal load, which can impair the optical properties of such reflective optical elements or can shorten the lifetime of such reflective optical elements, it is desirable to actively cool these reflective optical elements, for example with a gaseous or liquid fluid, for example water. To this end, at least one cavity, usually in the form of one or more channels, is formed in the reflective optical element in question, the fluid flowing through the channels in order to carry heat away from the optical element. In the reflective optical element according to the disclosure, however, the at least one cavity is not limited to the purpose of conducting a cooling medium.
Numerous production methods have been proposed, with which reflective optical elements can be produced with at least one cavity integrated in the optical element. DE 10 2005 053 415 A1 describes a method for producing individual mirror shells of a collector for EUV applications. In the method, first of all the optical layer, having the optically effective surface, of a mirror shell is galvanicly applied to a core, and subsequently a substrate is galvanicly applied to this layer, until the substrate has a desired thickness. After the core has been removed, a cavity is worked into the substrate from that surface of the substrate that faces away from the optical layer. The cavity is then filled with an electrically conductive material, and this is followed by a top layer being applied to the free surface of the substrate. The filling material, which was previously introduced into the channel(s), is subsequently removed again for example via solvents or by heating.
A potential disadvantage with such a production method is that the subsequent introduction of the at least one cavity into the substrate, i.e. after the substrate has been applied to the optically effective layer, can impair the optical properties of the optical layer. For example, the optical layer can be undesirably deformed in the process.
In some known production methods, it is proposed to apply prefabricated cooling lines in the form of tubes to the rear side of the optical layer of the mirror shell, for example by soldering, application by electroplating or the like. Such methods can be comparatively complicated.
Some conventional methods for introducing at least one cavity into a reflective optical element consist in producing the reflective optical element from two shells, i.e. from a base and a top, which are connected together by soldering or adhesive bonding or the like, as described in DE 10 2010 034 476 A1 or U.S. Pat. No. 6,792,016 B2. The at least one cavity was introduced into one of the two shells. In large-area optical elements, there can be a risk of at least partial detachment of the two shells from one another during operation especially when the fluid in the at least one cavity is under pressure and the optical element is operated under vacuum.
US 2006/0227826 A1 proposes introducing cooling channels into the substrate of the reflective optical element as radially directed bores after the optical element has been manufactured.
Known methods for producing a reflective optical element with at least one cavity for receiving a fluid can have the drawback that they are complicated and are not always compatible with the desired properties for high-precision optically effective surfaces, be these spheres, aspheres, free forms, or involve the use of materials which can be machined only with complicated methods and expensive tools.